linux-kernel - Re: [PATCH 3/3] rust: block: convert `block::mq` to use `Refcount`

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Message-ID: <9629a9d2-7e79-41ae-96c9-5fb26493c37a@gmail.com>
Date: Fri, 4 Oct 2024 18:43:57 +0200
From: Dirk Behme <dirk.behme@...il.com>
To: Gary Guo <gary@...yguo.net>, Andreas Hindborg <a.hindborg@...nel.org>,
 Boqun Feng <boqun.feng@...il.com>, Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <benno.lossin@...ton.me>, Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>, Jens Axboe <axboe@...nel.dk>
Cc: Will Deacon <will@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Mark Rutland <mark.rutland@....com>, linux-block@...r.kernel.org,
 rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] rust: block: convert `block::mq` to use `Refcount`

Am 04.10.24 um 17:52 schrieb Gary Guo:
> Currently there's a custom reference counting in `block::mq`, which uses
> `AtomicU64` Rust atomics, and this type doesn't exist on some 32-bit
> architectures. We cannot just change it to use 32-bit atomics, because
> doing so will make it vulnerable to refcount overflow. So switch it to
> use the kernel refcount `kernel::sync::Refcount` instead.
> 
> There is an operation needed by `block::mq`, atomically decreasing
> refcount from 2 to 0, which is not available through refcount.h, so
> I exposed `Refcount::as_atomic` which allows accessing the refcount
> directly.
> 
> Cc: Will Deacon <will@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Mark Rutland <mark.rutland@....com>
> Signed-off-by: Gary Guo <gary@...yguo.net>
> ---
...
> diff --git a/rust/kernel/block/mq/request.rs b/rust/kernel/block/mq/request.rs
> index a0e22827f3f4..7b63c02bdce7 100644
> --- a/rust/kernel/block/mq/request.rs
> +++ b/rust/kernel/block/mq/request.rs
....
> @@ -37,6 +38,9 @@
>   /// We need to track C and D to ensure that it is safe to end the request and hand
>   /// back ownership to the block layer.
>   ///
> +/// Note that driver can still obtain new `ARef` even there is no `ARef`s in existence by using

Do you like to check this sentence? Maybe:

Note that "a" driver can still obtain "a" new `ARef` even "if" there 
"are" no `ARef`s in existence by using ....

?

Best regards

Dirk